A blockchain is a digital ledger that records transactions across many computers in a way that ensures the information is secure and cannot be altered. Each transaction is grouped into a "block," and these blocks are linked together in a chronological order, forming a "chain." This decentralized structure makes it difficult for any single entity to control or manipulate the data. Blockchains are commonly associated with cryptocurrencies like Bitcoin, but their applications extend beyond digital currencies. They can be used for various purposes, including supply chain management, voting systems, and secure sharing of medical records, providing transparency and trust in various industries.